FIG. 8 shows a conventional structure of a rotor of a synchronous motor. The rotor of the conventional permanent magnet synchronous motor comprises rotor core sheets 71 laminated in the direction of and axis of rotor 70, permanent magnets 73 having substantially the same shape and same thickness as the slits 72 formed inside the rotor core thus laminated. Both ends of the laminated body have end plates 76 made of nonmagnetic metal, such as brass, stainless steel, or the like. All these components are assembled and fixed into a single piece body by caulking pins (not shown) piercing through caulk pin holes 75.
The synchronous motors have been driven on a power supply of 100V or 200V AC. Recently, however, some are driven on a low DC voltage, 50V or lower, for use in a car or the like apparatus. Such applications are increasing as a result of development in battery technology. Conventionally, when it has been driven on a 100V or 200V AC power supply, the motor coils are wound for some sixteen turns, seventeen turns, or more turns; in some motors they even exceed 100 turns.
The highest revolving speed of a motor has been controlled mostly by designing different number of turns with the coil. For example, when modifying a motor having 50-turn coil and the highest revolving speed of 9000 rpm to a motor having 10000 rpm highest revolving speed, the 50 turns are reduced to 45 turns. Since the highest revolving speed and the number of turns are roughly proportionate to each other, it is possible to adjust the speed by reducing the number of turns by 5 turns, in the above example.
However, in a motor driven on a low power supply voltage of 50V or lower, winding of the coil normally counts a mere several turns. Therefore, if a 50V motor having 5-turn coil and revolving at 9000 rpm, highest speed is reduced in the number of turns by 1 turn to 4 turns, the highest revolving speed can increase to reach a 11000 rpm or even higher. Thus, change in the number of turns by only 1 results in too much change in the revolving speed. Therefore, it is difficult to adjust precisely the highest revolving speed by means of the number of turns.
Thus, there is a need for a way to more precisely control revolving speed. The present invention addresses the above problem, and aims to offer an easy method of adjusting the highest revolving speed with the low voltage motors.
The present invention relates to a permanent magnet synchronous motor driven on a low DC voltage, and a method of its manufacture.
One aspect of the present invention is a low voltage DC motor having rotor, a stator or stator iron core, and a coil portion. The rotor has a permanent magnet. The stator iron core can have a plurality of tooth sections. The coil portion is wound around respective teeth no more than 10 turns.
In one embodiment, the motor has an end plate made of a magnetic material disposed at an end face or axial end of the rotor. The end plate can be a laminated body formed of thin sheets of a magnetic material, or a single magnetic steel sheet. In another embodiment, the rotor is fitted with a permanent magnet inserted in a slit formed therein and a magnetic substance is fitted in the slit, in addition to the permanent magnet. In another embodiment, a balance weight of a magnetic material can be disposed at an end face of the rotor or at an end plate (can be magnetic or non-magnetic material). In another embodiment, the length of the rotor in the axis direction is made shorter than that of the stator iron core. In another embodiment, the length of the permanent magnet in the axis direction is made shorter than that of rotor. In another embodiment, a magnetizing current applied at magnetization of the rotor is controlled. All of these features can be used to control the highest revolving speed of the motor. At least each of these features can constitute means for controlling the highest revolving speed of the motor.
Another aspect of the present invention is a method of manufacturing a motor, such as the motor described above. The highest revolving speed of the motor can be adjusted by disposing the end plate and/or the balance weight at the end face or axial end of the rotor and changing the thickness of the end plate or the size of the balance weight. Alternatively, the same can be achieved with the magnetic substance inserted into the slit formed in the rotor, or choosing the rotor length to be shorter than the stator length, or choosing the permanent magnetic length to be shorter than the rotor length, or controlling the magnetizing current applied at magnetization of the rotor.